The present invention relates in general to an automatic sewing arrangement for producing a stitch contour in a workpiece according to a predetermined program. A movably arranged workpiece receiving device is controlled with respect to the needle by means of a linkage cooperating with a control cam where the workpiece may be fed by the workpiece receiving device in any direction relative to the needle. In particular, the automatic sewing arrangement is installed with a sewing head producing a double lockstitch row where a throat plate with a special aperture for the needle is applied for effecting the interlocking of the needle thread and the hook thread.
In U.S. Pat. No. 3,808,991 there is described an automatic sewing arrangement of a type in which the workpiece is moved with respect to the needle by a control. Due to the different feeding directions of the workpiece with respect to the needle, the stitch formation is effected. The needle thread and the hook thread are regularly interlocked to each other or the needle thread is additionally turned around the hook thread. Due to such abnormally formed stitches an optical different appearance of the produced stitch row is achieved which negatively affects the quality especially at the production of decorative stitches. Furthermore, it is necessary for the formation of firm stitches having such abnormal interlockings, to increase the thread tension for the needle thread, thus causing an increase in thread breakage.
Generally, it is known from the employment of such automatic sewing arrangements having a multi-directionally and continuously working workpiece feeding movement with respect to the needle to especially round the edges of the needle aperture so as to achieve a stitch formation with as little thread tension as possible.
In the aforementioned U.S. patent it is proposed to eliminate the formation of such abnormal stitches by displacing the total workpiece feeding mechanism with respect to the stationary stitch forming arrangement.
From the German Pat. No. 285 751 a throat plate for a double lockstitch sewing machine capable of sewing forward and backward stitches is known. The aperture for the needle is formed at the upper side with a slot and at the under side with a tubular guide edge so as to control the hook thread. Due to the special formation of the aperture, the support of the workpiece in the next neighborhood of a penetrating needle is not possible, so that the sewing of relative soft fabrics of a workpiece is associated with difficulties. Furthermore, the slot-shaped extension of the aperture is connected to additional edges thus rendering not possible a multi-directional workpiece movement.
Accordingly, it is the main object of the present invention to profile the aperture for the needle in such a way as to support the workpiece to be sewn as close as possible to the position of needle penetration and to effect the stitch formation for eliminating abnormal stitch formation.
Another object of the invention is to construct an aperture guiding the threads in the aforementioned kind without exposing the passing threads to additional stress.
It is a further object of the present invention to create a throat plate with the aforementioned properties which is simple in construction and inexpensive to manufacture.
Still a further object of the present invention is to provide a throat plate with an aperture effecting the stitch formation regardless of the direction of feed movement which is easy to install in automatic sewing arrangements that are already in use.